The embodiments disclosed herein relate generally to the field of power generation and, more particularly, to a system and method for recovering waste heat from a plurality of heat sources having different temperatures for generation of electricity.
Enormous amounts of waste heat are generated by a wide variety of industrial and commercial processes and operations. Example sources of waste heat include heat from space heating assemblies, steam boilers, engines, and cooling systems. When waste heat is low grade, such as waste heat having a temperature of heat below 840 degrees Fahrenheit, for example, conventional heat recovery systems do not operate with sufficient efficiency to make recovery of energy cost-effective. The net result is that vast quantities of waste heat are simply dumped into the atmosphere, ground, or water.
Some power generation systems provide better reliability and off-grid operation with alternate fuels such as biogas or landfill gas, with examples being gas turbines and combustion engines such as microturbines and reciprocating engines. Combustion engines may be used to generate electricity using fuels such as gasoline, natural gas, biogas, plant oil, and diesel fuel. However, atmospheric emissions such as nitrogen oxides and particulates may be emitted.
One method to generate electricity from the waste heat of a combustion engine without increasing the output of emissions is to apply a bottoming rankine cycle. A fundamental rankine cycle typically includes a turbo generator, an evaporator/boiler, a condenser, and a liquid pump. However, water based steam rankine cycles are very unattractive in the aforementioned low temperature waste heat region due to high cost and low efficiency. The performance of an organic Rankine cycle (ORC) is limited by constraints of the working fluid circulated within the ORC. Any pure chemical used as a working fluid may be optimal for a specific range of cycle temperatures and pressures. It is therefore difficult to maximize the power output of a single fluid ORC for a system with waste heat sources of different temperature levels.
In another method to generate electricity from waste heat, single cycle system or two-cycle systems are used in heat recovery applications with waste heat sources of different temperature levels. Single-cycle configurations collect heat from the different waste heat locations in a serial arrangement of heat exchangers with an intermediate heating fluid. This “all-in-one” configuration lowers the maximum resulting fluid vapor temperature because the available heat from the various temperature level heat sources is mixed. An undesirable result of this configuration is a lowered Carnot efficiency. In such two-cycle configurations, the hot heat source heats a high-boiling point liquid in a top loop, and the cold heat source heats a low-boiling point liquid in a separate bottom loop. The two-cycle system generally achieves a better performance than a single cycle. Since components in the two-cycle system are more complex and require more components, the overall cost of the two-cycle system is significantly higher.
In another conventional system provided to generate electricity from waste heat, a cascaded organic rankine cycle system for utilization of waste heat includes a pair of organic rankine cycle systems. The cycles are combined, and the respective organic working fluids are chosen such that the organic working fluid of the first organic rankine cycle is condensed at a condensation temperature that is above the boiling point of the organic working fluid of the second organic cycle. A single common heat exchanger is used for both the condenser of the first organic rankine cycle system and the evaporator of the second organic rankine cycle. A cascaded organic rankine cycle system converts surplus heat into electricity within certain temperature ranges but does not recover waste heat over a wide temperature range.
It would be desirable to have a system that effectively recovers waste heat over a wide temperature range from multiple low-grade heat sources.